In recent years, air tightness of rooms in buildings have increased due to westernization, but the problems of dew condensation on wall surfaces and the like have occurred due to the loss of air permeability. In particular, the occurrence of water vapor due to use of space heaters causes a large problem of dew condensation in the winter. Such a phenomenon becomes a problem not only in rooms of general houses but also in buildings and storages used in operation. Specifically, there occur the problems with appearance due to dew condensation, floor stains due to dropping of dew condensation water from ceilings, stains on articles stored in storages, deterioration with water, and the like.
In refrigerators (or freezers) and refrigerator cars (or freezer cars), the humidity in refrigerators (freezers) is changed by circulation of cooling cold air and inflow of warm air due to door opening and closure and the like, thereby causing dew condensation. In vegetable compartments of refrigerators, water remains in the vegetable compartments to cause deterioration of freshness of vegetables. In refrigerators, dew condensation water produced on the inner walls of top plates drops on products (articles) during transport, thereby causing deterioration of appearance and quality.
The problem of dew condensation due to a change in humidity also occurs in casings for mounting, for example, electric lights or precision electronic apparatuses therein, thereby causing the problems of clouding of lenses, short circuiting of electric circuits, and the like due to dew condensation.
When dew condensation in such closed spaces is prevented, it is not sufficient to simply adsorb moisture with a drying material and the like. For example, in vegetable compartments of refrigerators, it is necessary to prevent dew condensation while maintaining appropriate humidity conditions suitable for storing vegetables. Similarly, in room spaces, it is important to maintain appropriate humidity.
Hygroscopic materials which simply adsorb moisture no longer can be used after moisture adsorbing performance is saturated. Materials having the moisture adsorbing and desorbing ability of adsorbing moisture at high ambient humidity and desorbing the adsorbed moisture at low ambient humidity can be continuously used over a long period of time because adsorption and desorption of moisture are repeated.
Humidity-conditioning materials which adsorb moisture at high ambient humidity to prevent dew condensation and which desorb the adsorbed moisture at low ambient humidity to increase humidity and recover hygroscopicity by moisture desorption are practically used as humidity-conditioning sheets mainly formed in sheets.
Conventional humidity-conditioning sheets including hygroscopic materials such as water-absorbing polymers, e.g., polysodium acrylate, have excellent hygroscopicity but have low moisture desorbing ability, and thus the sheets have the problem that they little desorb moisture under usual operation conditions or if they desorb moisture, a long time is required until the initial hygroscopicity is returned because of the low moisture desorption rate.
When humidity-conditioning sheets with low moisture desorption are continuously used, the water adsorbed in the sheets is saturated, and the sheets no longer function as humidity-conditioning sheets, thereby failing to prevent the problem due to dew condensation.
Humidity-conditioning sheets having insufficient moisture desorbing function also have the hygienic problem of occurrence of molds or bacterial growth due to water retained over time.
Therefore, humidity-conditioning sheets are required to have the function to reversibly rapidly adsorb and desorb water vapor.
Examples of conventional humidity-conditioning sheets include one proposed in Japanese Unexamined Patent Application Publication No. 2000-43179 in which a base sheet is coated with a mixture of template-type meso-porous silica having a controlled average pore size and a binder to form a humidity-conditioning layer. However, the humidity-conditioning sheet has the humidity-conditioning layer formed by coating and thus has limitation on thickening, and thus the excellent humidity-conditioning function inherent in the meso-porous silica used as a humidity-conditioning material is not effectively utilized.
Japanese Unexamined Patent Application Publication No. 2001-334596 proposes a humidity-conditioning sheet having a humidity-conditioning layer formed by bonding silica gel particles between two sheets with hot melt powder. The humidity-conditioning sheet also does not have sufficient humidity-conditioning properties.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-43179
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-334596